1. Field of the Invention
This invention relates to a system for controlling a direct-coupling clutch of a torque converter of an electrically controlled automatic transmission.
2. Description of the Prior Art
It is known in the art to lock up a torque converter upon upshift to 2nd, 3rd or other predetermined gear positions by supplying an operating oil pressure (line pressure) to a direct-coupling clutch control circuit through a shift valve as soon as the predetermined gear position is reached, cutting off the supply of the oil pressure to the direct-coupling clutch control circuit in lower gear positions. As illustrated in FIG. 1, in a direct-coupling clutch control for an electrically controlled automatic transmission, the line pressure which is supplied to a conduit 1130 in the lock-up gear position is drawn through an orifice 4220 into a conduit 1140 with a solenoid valve 4200. The solenoid pressure which is controlled by the solenoid valve 4200 is applied to a spool 2920 of a lock-up control valve 2900 at one end loaded with a biasing spring 2910, against the line pressure in the conduit 113 acting on the other end of the spool 2920, to connect either a clutch engaging conduit 1220 or releasing conduit 1210 of a direct-coupling clutch 500 of a torque converter with a conduit 1200 which is in communication with an oil pressure source, thereby engaging or releasing the direct coupling clutch 500. When the oil pressure is supplied to the direct-coupling control circuit upon an upshift to a lock-up gear position, it is often experienced that the lock-up control valve 2900 is actuated to lock up the torque converter irrespective of the ON- or OFF-state of the solenoid valve 4200 due to a time delay which is caused by differences in the length or resistance of the oil conduits. Especially in a case where an orifice is provided in the conduit 1130 in order to generate a solenoid pressure by controlling the line pressure by a solenoid valve 4200 as shown in FIG. 1, there occurs a delay in the supply of the oil pressure to the conduit 1140. For example, in a case where the line pressure is supplied to the conduit 1130 through a 1-2 shift valve when the latter is shifted to 2nd position to lock up the torque converter in 2nd and higher gear positions, the spool 2920 of the lock-up control valve 2900 is shifted to the upper position in FIG. 1 as soon as the line pressure is admitted into conduit 1130 upon upshift to 2nd even if the solenoid valve 4200 is in OFF-state, causing a premature lock-up of the torque converter (without control by the solenoid valve 4200) which may lead to an engine stall or other dangerous accidents.